1. Field of the Invention
The present invention relates to a technology for detecting image information from pattern images in an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a copier, a facsimile machine, a printer, or a printing machine, a visual image carried on a photosensitive element is transferred to a transfer member.
A recording sheet that directly contacts the photosensitive element or a belt-type transfer member is used as the transfer member. The belt-type transfer member is used to form a multi-color image.
To form a multi-color image, the image forming apparatus employs photosensitive elements. On each of the photosensitive elements, a latent image of a different color is formed. A belt facing the photosensitive elements is rotated. The belt functions as an intermediate transfer member or a conveying member that caries a recording sheet on a surface (refer to Japanese Patent Application Laid Open No. H10-161388).
When the belt is used as the intermediate transfer member, each image formed on each photosensitive element is sequentially transferred and superposed onto the intermediate transfer member, by a primary transfer process. The superposed image is then transferred to a recording sheet by a secondary transfer process. When the belt is used as the conveying member, a recording sheet is carried on the surface of the belt facing the photosensitive element. As the belt rotates, images formed on each photosensitive element are sequentially superposed on the recording sheet.
In an image forming apparatus used for forming multi-color images, image quality, such as color reproducibility, needs to be stabilized. There is a method of stabilizing image quality by forming pattern images to detect image density, as disclosed in Japanese Patent Application Laid Open No. H10-161388.
Specifically, pattern images are formed on the photosensitive element or the intermediate transfer belt, and the pattern images are optically read. Based on results obtained by reading the pattern images, a feedback control is performed to control various parameters of image forming conditions.
The feedback control is performed as follows. An image-density detecting sensor detects an amount of toner adhering to a pattern image formed on the intermediate transfer belt. When the amount does not satisfy a predetermined condition, various parameters are controlled to satisfy the condition. The parameters include a writing output property, a charging property of the photosensitive element, a charging property that affects adherence of the toner in a developer, and a developing bias property that controls the amount of toner adherence.
The pattern images formed on the intermediate transfer belt are larger than a detection area detected by the image-density detecting sensor. Density of a pattern image that covers the entire detection area is measured. Based on the detected density, the amount of toner adherence is calculated. The calculated amount is used to determine whether the predetermined condition is satisfied.
The pattern images are formed in an area other than a regular area in which a regular image is formed so as not to overlap a starting end of the regular area in which a next regular image is to be formed. A secondary transfer device is separated from the intermediate transfer belt while density of the pattern images is detected (refer to Japanese Patent Application Laid Open No. 2000-123052).
Moreover, an optical senor facing the intermediate transfer belt at a portion stretched out in a circumferential direction is used to detect the density (refer to Japanese Patent Application Laid Open No. 2002-123052, Japanese Patent Application Laid Open No. 2003-167394).
In the conventional technology, as disclosed in Japanese Patent Application Laid Open No. H9-204108, the detecting sensor is provided at a downstream side of a primary transfer position of the intermediate transfer belt and an upstream side of a secondary transfer position. However, this layout is disadvantageous in that the detecting sensor faces upward and toner scatters on to the detecting sensor. Moreover, because a sufficient distance is required between the primary transfer position and the secondary transfer position, it is difficult to reduce a size of the image forming apparatus, and to reduce time required to complete print of the first page.
On the other hand, if the detecting sensor is provided at a downstream side of the secondary transfer position, a secondary transfer roller needs to be applied with a bias of the same polarity as that of the toner when the image patterns pass through the secondary transfer position, as disclosed in Japanese Patent Application Laid Open No. H7-253729. However, it is impossible to completely prevent the toner from transferring to the secondary transfer roller. Moreover, an amount of the toner transferring to the secondary transfer roller is affected by the environment. Thus, the toner soils the surface of the secondary transfer roller, and the soiled secondary transfer roller soils a rear surface of a sheet of transfer paper. Moreover, irregularities in pattern images might be caused, resulting in inaccurate detection of the image density. One approach is to separate the secondary transfer roller from the intermediate transfer member. However, when the pattern image is created in between regular images being printed out continuously, such an action of attachment and detachment of the secondary transfer roller causes undesired variations in rotation of the intermediate transfer member. This has a detrimental affect on the images.
Another approach is to use a non-contact-type secondary-transfer device such as corotron. However, this increases ozone emission, and is disadvantageous in terms of conveyability of transfer paper.
In a technology disclosed in Japanese Patent Application Laid Open No. 2002-123052, the secondary transfer roller contacts with and separates from the intermediate transfer belt for detecting the pattern images. Accordingly, extra time is required to be provided for such movement. This requires larger intervals between recording sheets being conveyed on the intermediate transfer belt. As a result, image processing takes longer time.
Moreover, an impact of the secondary transfer roller due to such movement causes the intermediate transfer belt to shake. This affects an optical distance between the pattern images and the detecting sensor, resulting in detection errors. To overcome this problem, formation of pattern images is delayed from when regular images are formed, as shown in FIG. 16. The image forming process for regular images is suspended, and the secondary transfer roller separates from the intermediate transfer belt, before density of the pattern images is detected. In this manner, the detection process is unaffected by the shaking of the intermediate transfer belt. However, it takes a significantly long time for suspending and resuming the image forming process. If the optical sensor is positioned facing the portion of the intermediate transfer belt stretched out in the circumferential direction (refer to Japanese Patent Application Laid Open No. 2002-123052 and Japanese Patent Application Laid Open No. 2003-167394), the impact of the shake of the secondary transfer roller is particularly large.
A cleaning device can be provided to remove toner adhering to the secondary transfer roller after the secondary transfer process. However, the cleaning device is not provided when space and costs need to be saved. When the cleaning device is not provided, the above-described contacting/separating mechanism is required. However, usually, no means for solving problems caused by the shaking of the intermediate transfer belt is provided.
Pattern images are formed to provide image information on each color. Therefore, all pattern images need to be formed on the intermediate transfer belt before the secondary transfer process begins. However, when the secondary transfer process is brought forward in order to save time, a pattern image of a last color might not yet be formed. Thus, depending on timing in starting the secondary transfer process, the pattern images cannot be properly formed.
When performing the feedback control in an image forming apparatus including more than one image forming unit and the intermediate transfer member, pattern images with different amounts of toner adherence are formed by changing image forming conditions. It is difficult to perform a regular image forming operation during the feedback control. Thus, copying and printing operations need to be suspended while the feedback control is performed.
The time of the feedback control needs to be minimized to reduce a downtime during which copying and printing operations are suspended.